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Why would flares be fired into the air after a flight lands?
On Monday 23rd September at about 14:30 BST my plane landed at London Stansted. Soon after landing, while the plane was at the end of its deceleration, I noticed what appeared to be two flares or fireworks shot about 50 ft into the air above the grass to the right of the runway (I believe it was runway 22).
What on Earth could that be about? The flight was Ryanair FR587 (and the plane's registration was EI-DYL if anyone is interested).

One possibility is scaring away birds, although I have no idea if it's possible to confirm that or not.

Bird-scaring flares at Birmingham Airport

@user52723a That is what it looked like (although I couldn't see where they were being fired from)

Checked your flight at FlightRadar24. There was no such flight as FR587, but there was a flight EI-DYL. It was FR4030. Could that be you're aircraft?

@FallenUser It was FR587

Unfortunately, FlightRadar24 cannot replay any of the flights of the aircraft or even find the flight.

@FallenUser I don't think it will help to answer the question, anyway

Possible duplicate of What are the ways to keep the birds away from airfields?

I know that flares are used at Dover air force base in Delaware sometimes to deter birds from the airfield. I also know that flares are just a common bird deterrent anyway. So it may have been simply to clear the taxiways and runways of birds before the next departure or arrival.


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Are there tricks or training to handle spatial disorientation?
For spatial disorientation, are there tricks to block out certain inputs like focusing on what the eyes see and blocking out the ears? I am curious if there are training techniques to practice on the ground to help with spatial disorientation. Do you have any techniques to practice?

AFAIK the trick to blocking spacial disorientation is "trust the instruments not your senses"

The FAA's Instrument Flying Handbook chapter 3 might give you some ideas. But it mostly comes down to having a source of information that isn't affected by disorientation - i.e. instruments - and relying on it instead of on your senses.

@Jamiec Yeah, sure, but I think what the OP wants to know is "how do you train not to trust your senses ?"

In flying you never trust your inner ear because it is easily deceived, you can't block out the inner ear inputs, you simply learn to ignore them in favor of your eyesight. When you first learn to fly it is in visual meteorological conditions using the horizon as a reference, then in instrument conditions (if you choose to get an instrument rating) using your instruments such as the Attitude Indicator (AI) and the Turn Coordinator (TC).

Thank you! When I flew, I used the horizon. I remember in our simulator room that there was an issue with motion sickness. I cannot remember if we painted in a horizon or whether there was one that we had to remove. Now, I know who to ask. Thanks, again!

The issue with spatial disorientation is that sensory input is insufficient (based primarily on vestibular input) or that the pilot fails to properly process sources of valid orientation.
Generally, when we refer in aviation to spatial disorientation, we really mean attitudinal disorientation.  (Spatial disorientation could also mean not knowing where we are relative to a fix or navaid, as an example. Rather we are really interested in pitch and roll.)
Primary input for attitudinal orientation is an artificial horizon, which emulates a visual horizon providing pitch and roll information. Secondary input can come from a combination of altimeter, turn indicator, and variant instruments (VSI, DG).
With respect to techniques and practice, the standard method is developing a discipline to rely on the AI and associated instruments, and to force a reliance on the instruments, particularly when the pilot senses a conflict. Some people acquire this discipline quickly and others take longer. The general standard for teaching is to continue to provide practice and diverse evaluation opportunities, until the pilot demonstrates consistent correct behavior.
Assuming primary and instrument students are acquainted with the basics, I tend to employ frequent "unusual attitude" opportunities. My experience has been that students who perform well under stress, generally perform well without stress. Unusual attitude opportunities can be instructor induced opportunistically, such as when flying to and from the practice area. When putting on foggles, I customarily will have the student raise their head up after having "reoriented" the aircraft. This may happen 4 to 10 times, sometimes more, in a lesson, and generally students get very proficient.
An alternative technique, which is often done on student cross countries, is to have the student close their eyes, and continue to try to keep the plane straight and level, and then command them to open their eyes once some attitudinal change has presented itself. (There is a secondary benefit to this method, and that is it encourages proper trim maintenance.) If the student has a view limiting device on (foggles) then they get the opportunity to recover using instruments. If they do not have a view limiting device, then they can benefit from both instruments and outside reference.
These things can be practiced with a simulator, however, unless you have a Level D simulator (full motion) there is little opportunity for false vestibular input, and there is little benefit. Besides, nothing beats pulling 1.5Gs and hearing the wind pick up, and sensing the sunshine moving around in the cockpit to get a student's interest up prior to opening their eyes to begin the recovery.
So the best answer to practice is to have your CFI provide frequent, varied, and sometimes stressful opportunities. In my experience, I have never had a student who could not master recoveries, even with nausea inducing preludes.

It is possible to induce spatial disorientation on the ground by using a small funnel or plastic syringe without a needle attached to instill cold or warm water into one ear only. this will generate vertigo similar to spatial disorientation. We did this in medical school in order to understand vertigo related to the otolith system. Couple this with a simulator and perhaps this could be a useful training tool.

Interesting answer. Has any pilot tried this and could give some feedback?

Yes there are spatial disorientation trainers, such as the one I've worked on that was built in Taiwan for a flight-medical research institute. It is mounted on top of a slip-ring that allows the cabin to spin: a problem with spin is that pilots can recover from it, but it creates sensory disorientation which can induce a second spin from which recovery is much harder.

The cockpit inside has both a jet and helicopter set-up, it is used for demonstrating the effects of sensory disorientation and for desensitisation of motion sickness.

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How do door seals work?
The pressurization in the cabin requires the doors have their seals working properly. How do they prevent pressurized air from leaking from the doors?

Same way any other type of seal works, the sealing material creates enough of a barrier between one side and the other to prevent pressure to leak through the seal.

Think cork plug in the inside of the neck of a bottle instead of on the top of it.

How do they prevent pressurized air to leak from the doors?

They don't. The seal does not have to be perfectly airtight.
The requirement is that it will create enough impediment to the air that wants to gush out, where "enough" is defined by the capability of the conditioning system to input new air into the cabin.
In other words, everything is fine as long as the air mass that gets out of the doors is less than the maximum air flow that the air conditioning system can input into the cabin.
Note also that, as stated on Wikipedia, aircraft are equipped with one (or more) "out-flow valve(s)" and "pressure relief valves": these valves intentionally let air out of the cabin to avoid over pressure in the cabin and thus they prevent possibly dangerous scenarios.

darn, I'll never learn. Thanks @Simon.

Note that while small leaks don't cause pressure problems, they can still cause very unpleasant whistling sounds.

@Federico Think nothing of it, that's why I left no comment. Your English is many times better than most users on here are with Italian or Spanish, me included (sorry, don't know which one).

Like any pressure seal they are generally rubber seals. You can find one of Boeings patents here which covers door seals. Some smaller planes (and unpressurized GA planes) use inflatable seals, the door is closed and then the rubber boot is inflated to form a tighter seal. You can find an interesting discussion on the seals here but in general they may have tiny leak issues. This was the cause of at least one A380 diversion recently, but it looks like airbus is working through it and has a solution. Since the cabin has an outflow valve to keep it properly pressurized a tiny leak in the door will be compensated by the outflow valve. This airbus briefing warns that they can be hazardous and hard to detect if one does occur the crew may not know until the O2 masks deploy.

Slow/Insidious decompression involves a very gradual decrease in cabin
  pressure. Slow decompression may be the result of a faulty door seal,
  a malfunction in the pressurization system, or a cracked window.
Slow
  decompression may not always be obvious. The cabin crew may not notice
  the changes in the cabin, until the oxygen masks drop down from the
  Passenger Service Units (PSUs).

It has been noted that leaky seals on both pressurized and unpressurized doors can lead to a very loud noise in the cabin.
The seals do at least to some level try to keep water out. This is mainly because excess water may build up and then freeze as the airplane climbs. This ice build up in the door could in theory cause an issue (although I cant find any cases where it has).

Looks like that A380 diversion may have been more than just a leaky seal. (I trust AVHearld more than crikey.com.au...)

Very Correct, If you dont mind ill use your link instead of the one I had. I know it at least prompted a door seal repair.

@FreeMan Thanks for the link, interesting stuff! "After the aircraft had levelled off at cruise altitude FL370 the supervisor informed the flight crew, who observed the cabin pressure at 6000 feet." <-- Is that an over-zealous application of the sterile cockpit rule? I would think that "the door may be loose" is something a pilot would want to at least be aware of during takeoff.

@yshavit, it's likely the flight took off below the noted cabin altitude of 6000', therefore, there would be nothing for the pilot to notice at takeoff. i.e. cabin altitude ≈ runway altitude. I'm not sure what the cabin altitude of an A380 would normally be, but I'm guessing it's lower than 6000', thus triggering concern with the cabin crew.

@FreeMan I was actually more looking at the "after the aircraft had leveled off at cruise altitude" bit. Why wait that long? My only guess would be because of the sterile cockpit rule.

A lot of aircraft with pressurised cabins have plug type doors which press against the fuselage when in closed position. There is a (silicone) rubber seal where it touches against the fuselage, which minimizes air leak. The image below shows the seal in an aircraft door.

Image from aviationtroubleshooting.blogspot.in

The door in the picture clearly are not plug type, since the seal is on the inside.

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How far back in time does FAA Institutional Authority for 61.160 (R-ATP) minimums go?
61.160 contains the requirements for an R-ATP certificate, and there is a list of FAA approved institutions that are eligible to graduate applicants to meet the R-ATP requirements.
There is a column with issue and revision dates:

The increased qualification requirements were enacted in August 2013, and therefore these schools' LOAs are all issued after that date.
If a pilot had graduated from one of these schools' programs prior to the issuance of the LOA, does it still grant them the eligibility for reduced minima?  Is the authority retroactive?  If so, by how much time?

I expect the answer to your question is in the LOA. LOAs are unique to the authorized entity. While they are similar, they can vary significantly. Thus the notes at the bottom of the list.  Specifically: Contact your institution for more information concerning their authorization.  and the following: NOTE: Institutions that do not wish to include a certifying statement on their graduate’s official transcript must include the certifying statement in a separate document. This document must be submitted as part of the application.

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Moment Coefficient Variation with Angle of Attack
looking at the moment coefficient for certain NACA profiles, I noticed that their graph changes with AoA and is not a constant horizontal line as I would expect (pure moment independent of the AoA). This seems to imply that the moment and the associated moment coefficient is measured relative to a point $P$ (maybe the leading edge $LE$?) that is not the aerodynamic center $AC$ (otherwise the moment would be independent of Aoa) but I am not sure what that reference point may be. The figure below illustrates what I discussing...

Its most likely from the AC. When it is said the moment does not change with AoA in AC it does not imply a completely straight line, it can vary a bit but is somewhat straight as yours also is around 0.1. And that 'rule' only goes for low speeds btw. Once AoA is increased above a certain point when separations effects come into play, it will no longer be 'straight' as your graph also shows around 8 degs.
If you think the line is not straight enough, maybe your AC is not calculated exactly (maybe it just uses the rule of thumb of 1/4 chord).

I see. Thanks invariant. I guess I was expecting an unrealistic straight horizontal line. This graph is from the internet for a certain NACA profile (I didnt calculate it)...T

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Does the direction of rotation of the propeller affect the direction of the thrust generated?
Does changing the direction of rotation of the propeller from clockwise to anti-clockwise change the direction of thrust it generates?

What is your understanding of how a propeller works? How do you think it generates thrust at all?

Possible duplicate of Why don't planes have a reverse propeller as air brakes?

It depends.
For that simple kind of a fan with flat blades, yes. For anything that was ever installed on a full scale airplane, it will generate no or very, very little thrust, more likely backward.
The blades of a propeller are airfoils—like wings—mounted with suitable angle of incidence and usually slightly cambered. Any efficient airfoil is necessarily designed for one direction. If you move them backward—by turning the propeller the other way than it's designed for—they will generate very little lift. And the remaining bit may be cancelled by the camber, because the incidence will be for generating thrust backward, but the camber will still favor forward thrust.
For much more details, see
Is there an aerofoil that gives reasonably good lift for both air flow directions: forward and backward?
Only flat plates do generate lift both ways, so a simple fan with flat blades would indeed generate backward thrust. But it's not efficient in either direction in the first place, so it may be appropriate as a ceiling fan, but definitely not as aircraft propeller.


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What does it mean for a plane to be aerodynamically stable?
I've heard that most planes (excepting fighter jets) are "aerodynamically stable." What does that mean?
Let's say you're cursing around in a Cessna 172 without autopilot, and you take your hands off the controls. What happens?

Will the plane stay at the same pitch and avoid stalls and dives?
Will it stay at the same altitude?
Will it say at the same heading?
Will it correct each of these as the plane is buffeted by turbulence?

Didn't your parents tell you not to curse in a 172?

@copper.hat wow!! I think I might leave that typo in there cuz I find it so funny...

Do! It creates a wonderful visual :-).

Haha, I absolutely love that image, definitely leave it

"Let's say you're cursing around in a Cessna 172" Sounds like some of the guys I hear on the CTAF around here.

Related: Are fighter jets designed to be so inherently unstable that a human can't fly one unassisted?

There are numerous cases of light aircraft that weren't correctly chocked during hand starting taking off and flying for a prolonged period (sometimes hours) with no one aboard.  Not to mention free flight models, which are built for stability, have been recorded to fly more than an hour in thermal lift before flying out of sight or falling out of the lift.

Static stability
is the tendency of a system to return to its initial state after a disturbance. Typical disturbances in case of airplanes are:

Flying into a vertical or horizontal gust
A jerk on the stick

The classic explanation is with a ball sitting in a pit. Whenever its position is changed by a disturbance, it will roll back towards the center. This does not mean that it will stop there - in an ideal world free of friction it will keep moving back and forth like a pendulum.

This is static stability. Broadly speaking, static stability is achieved by placing the center of gravity ahead of the neutral point, the point in which all additional forces due to a change in angle of attack can be summarized. There are two neutral points, one for longitudinal stability and one for directional stability. In both cases the change in angle of attack or sideslip will create a correcting moment around the center of gravity, pulling the aircraft back on its old path.
Even altitude stability can be achieved, but here we take advantage of the fact that air becomes less dense the higher you climb. If the aircraft is trimmed for horizontal flight at a certain altitude, an altitude change will mean that the power setting does no longer match drag at this new altitude. The aircraft will either climb or sink, depending on the altitude change, until the old altitude has been reached again.
In all cases you will experience overshoots and oscillations around the trim point. It can even happen that these overshoots become worse the longer the oscillation lasts (the phugoid motion in a glider is a good example). To stop the oscillations, you need to add
Dynamic stability
which describes the behavior of the aircraft over time. In most cases, friction will ensure that motions die down, and sometimes aircraft need little helpers like yaw dampers to get those under control.
You mention stalls and dives: They can indeed occur in a statically stable airplane. Low drag means low damping, therefore many high performance gliders have a dynamically unstable phugoid motion. I know, Wikipedia spells this wrongly, but the explanation is OK, so I linked it nonetheless. If you wait long enough after an initial upset, the oscillations will become so severe that the aircraft stalls at the upmost point of the cycle.
If you keep enough altitude and have no traffic nearby, this is fun to try.
Spiral dive
Note that I omitted to say something about roll stability so far. The glider (and your Cessna 172) will also start to roll, and in many cases the roll motion will increase more quickly than the phugoid, so you might need to try several times before you get a phugoid-induced stall. Most aircraft have a weak tendency to increase roll angle and to eventually fall into a spiral dive.
There is no aerodynamic mechanism to right the aircraft up after a roll disturbance. Sorry, @kevin, but dihedral won't help - it only works in case of sideslip.

I'm not quite clear why there would be no possible aerodynamic mechanism to right an aircraft after a roll disturbance.  If one were to drop a plane with a "V"-shaped wing, then (once the plane started falling) whichever side was lower would experience more lift, would it not, establishing an equilibrium where both wings were equally high?  What would prevent such a principle from establishing roll stability in flight?

@supercat: There is an inertial mechanism, but no aerodynamic mechanism. Your plane will be helped by dihedral to fly coordinated turns, that's all. If you want to know more, post another question.

@supercat: The upwards component of lift, which is the one we want for keeping the plane in the air, is indeed largest for the lower wing, but the component perpendicular to the line between that wing's center of lift and the roll axis, which is the one that makes the plane roll, is the same for both wings.

I edited this and replaced all instances of "phygoid" with "phugoid", which I think are the respective German and English terms for what I was always taught to just call "Long Period Mode". I hope you don't mind.

@AEhere I do mind indeed. "Phugoid" is wrong - the word comes from ancient greek and the correct spelling should be "Phygoid". That the English literature is using the - in my eyes - wrong spelling should not mean that I repeat the same mistake blindly. German, by the way, uses the correct spelling.

@PeterKämpf I totally see where you're coming from, but of all the sins of English, I'm willing to give this one a pass, if only because they have been consistent about using a dodgy spelling for the past century: https://en.wiktionary.org/wiki/phugoid Thanks for making me look deeper into this one.

@AEhere The French Wiktionnaire is a bit more explicit and says about Lancaster that he was "victime de sa mauvaise connaissance du grec" (victim of his poor knowledge of Greek)

Thanks for the link, that does explain things a bit. I can roll the edit back if you wish, though perhaps this would be best included as a note in the answer?

@AEhere: In any case, "phugoid" being the commonly-used spelling inherently makes it the correct spelling; that's how language works.

Your guesses are pretty correct - an "aerodynamically stable" aircraft tends to stay (relatively) straight and level if the controls are let go.
Pitch
Let's say the aircraft's elevator is trimmed to fly level (maintaining the same altitude). You push on the yoke to lower the nose, then release the pressure on the control. The nose-down altitude allows the aircraft to pick up more speed. As speed increases, the wing generates more lift, and the aircraft slowly pitches up. If you pull on the yoke then let go, the nose-up altitude will slow the plane down, which reduces lift. When lift is reduced, the nose drops, picking up speed. A similar argument applies if the plane's pitch is altered by a gust.
This stability is called Longitudinal Stability. It is closely related to the forward/aft position of the Center of Gravity (CG). An aircraft with an aft CG has less longitudinal stability.
Roll
Similarly, the ability of the plane to level its wings when banked is called Lateral Stability. If you roll the plane right 10 degrees then let go, it has a tendency to slowly roll left, going 7 degrees right, 5 degrees right, eventually to almost level.
A Wing Dihedral is a design which adds lateral stability.

Yaw
This is called Directional Stability. Similar to pitch and roll, it is the tendency for an airplane to recover from a disturbance in the yawing plane.
Stability vs maneuverability
The more stable an aircraft is, the less maneuverable. This is a natural trade-off following the laws of physics. Cessna 172s are very stable, suitable for student pilots. But you can't make a Cessna 172 do very rapid changes. Aerobatic aircrafts and fighter jets can respond to pilot inputs very rapidly, but controlling one requires much more skill.
*On single engine propeller planes like the Cessna 172, the torque spinning the propeller (usually clockwise) tends to roll the aircraft counterclockwise (i.e. left) if the controls are fully released. This is due to the "action and reaction" physics principle.

Thanks! And for altitude, the pitch is the relatively constant so vertical speed should stay around 0, right? But if there's turbulence or something and the plane drops 200ft, is there anything that makes a plane "want" to return to it's cruise altitude or will it stay at the new altitude?

Planes fly "iso-pressure". What that means is, flying at a specific "altitude" is really flying at along a constant atmospheric pressure. It's very seldom a plane drops literally 200 feet in space. Rather, it travels horizontally level, but due to a rapid change in the atmospheric pressure, the altimeter reading jumps 200 feet. A dropped altimeter reading means the planes enters a higher pressure region, but flying at the same speed it was moments before. There's excess lift, so it'll climb.

You are very optimistic when it comes to roll stability. Most aircraft will slowly diverge into a spiral dive. Try it!

@PeterKampf edited the roll angle to 10. Yea at 30 you may need counter aileron to keep it from rolling too far.

Normally you don't need roll input, just patience, to experience a spiral dive. Dihedral will not help, btw.


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Do open propellers experience pre-entry drag (additive drag) like an engine in nacelle?
Engines in a nacelle experience pre-entry (also called additive) drag.
Do unducted propellers experience it too?


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How does gust affect a swept wing
I have a basic question that I cannot seem to find a solid answer for:
How does a gust affect a swept wing?

with the cosine of the sweep angle. A 60° sweep will halve the loads from gusts.

What kind of effect are you asking about? It is easy to assume structural, but we can't read your mind.

Sorry our experts did not weigh in on this one in more detail.
Sweeping the wing increases the "dihedral effect" of tending to create a greater roll force when struck with a side gust, as the windward wing is angled towards the gust, and the leeward away.  This is exactly the same effect as what happens in a side slip.
Swept wings also improve yaw stability.
Of course, a very sharply swept wing presents a shorter torque arm to a rolling gust than a less swept wing of equal length but greater span.
The "dihedral effect" of swept wings can be aerodynamicly countered with side area under the CG, particularly fuselage side area or landing gear spats.  Ventral fins offer another alternative.
A gust from the direction of flight will affect a swept wing similarly to a straight wing, producing a momentary increase in lift.
The venerable Tiger Moth biplane has slightly swept wings.

Definitely some good points here; original question was overly broad.


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How much time does it takes to buy a landing slot?
According to Wikipedia, landing slots are sold for several million dollars at busy airports such as London Heathrow. Given this high value, such transactions may take weeks to conclude. Moreover, some slots can be free quickly and unexpectedly (e.g. when a company cease activity).
My question is: for the busiest European airports (Heathrow, Schiphol, Charles de Gaulle,...), how much time does it take to complete these steps (and the steps I cannot imagine):

The airline decides to open a new route
The airline identifies the slot they'll buy
The airline begins negotiations to buy the slot
The airline acquires the slot
The airline can operate this slot
The airline begins to use this slot

@bogl you should know by now that not everyone on the internet is fluent in English, and that you can edit posts to improve them.

@AEheresupportsMonica I would have edited it, if I was certain about the author's intention.

This is a fascinating question!  I wonder if there's a b2b eBay for such things ?  :O

I know that in the Netherlands this procedure is open and not obscured behind a login portal.
In the Netherlands the commercial airports are operated by the Schiphol Group, the number of flight movements or slots per airport is determined in collaboration with the government. For Schiphol this is now 500,000 per year. The allocation of the slots is governed by a second body which is set up to be non discriminating in the allocation of slots. Most of the answers to your questions can be found there: https://slotcoordination.nl (English website)
Who allocates slots?

Under the European Union rules – (EC) Regulation 95/93, as amended -,
  a Member State of the EU needs to appoint an airport coordinator for
  each coordinated airport. The same coordinator can be appointed for
  more than one airport. SACN has been appointed for coordinating AMS,
  EIN and RTM. Coordinators are independent bodies to maximise
  transparency.

How often are slots allocated?

Slots are allocated for each season, and slot coordination recognises
  two different ones. There is a Winter season and a Summer season. The
  mark of the change of season coincides in the EU with the change of
  summer/winter time. The difference is recognised to make sure we use
  daylight optimally.

How are slots allocated?

The allocation planning process starts around six months before the
  start of each season. Slots are allocated free of charge from a pool
  of available slots. Allocation takes place according to certain
  priority rules in a neutral, non-discriminatory and in a transparent
  manner. The leading principle of distribution is based on historical
  precedence, the so-called grandfather rights. When an airline operates
  a series of slots in a season for 80% or more, it can hold on to the
  same series in the next same season. When it operates less than 80% it
  loses its historic rights (the so called ‘use-it-or-loose-it’ rule).
  Slots can be re-allocated until the day of operation.

What happens when the requested slot is not available?

An airline can keep its request on the list of ‘outstanding requests’
  to see if during the allocation process slots become available over
  time. Airlines can freely exchange slots. Prior to each season a
  worldwide Scheduling Conference takes place enabling face to face
  meetings between airlines, airports and coordinators to improve the
  allocation process.

Apart from being assigned a slot, an airline also needs to file a request with the airport to request for the required services to handle the aircraft. Schiphol has a very informative website with a lot of publicly available information:
For passenger handling (gates, check-in, etc.)
https://www.schiphol.nl/en/operations/page/passenger-process/
For aircraft handling (gate assignment procedure etc.)
https://www.schiphol.nl/en/operations/page/aircraft-process/
If you want to know more about the trade in landing slots then I would suggest opening a new question since there is a lot to tell about that subject as there is a lively trade going on there.


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Why do the flap maneuvering speed margins to stick shaker depend on altitude?
Why do the flap maneuvering margins to stick shaker reduce with increasing altitude?
Ref: It is written in FCTM of the Boeing 737-800:

It's because the air is thinner, meaning the dynamic pressures are reduced for a given velocity through space, but the inertial mass of the air frame is the same as at sea level. Or you could flip it over and say the inertial effects (true airspeed) are higher for the same indicated airspeed (dynamic pressure). In other words, the dynamic forces have less influence over the inertial forces for a given velocity and mass. As well, the thrust margins to accelerate out of a stall are also less.
Best illustration is to take it to an extreme.  Say you are at 45000 ft, flying near stall at 150kt indicated.  That is, the dynamic pressures of the air are equivalent to 150kt at sea level. However, your actual velocity through space is roughly 300kt, so all of the inertial forces are effectively doubled relative to the aerodynamic forces, and on top of the that, you have more or less zero thrust reserve. You now have WAY less stick shaker margin, and a stall at that altitude requires extreme responses (aggressive dives to gain speed).
In your case, the effects are a lot more subtle, but still significant, especially approaching 20000 ft.


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Why is the King Air avionics switching system wired normally closed?
In the King Air 200 and B200 (perhaps other King Air models as well), the avionics electrical bus relays are normally closed. With the Avionics Master switch in the "OFF" position—which is electrically closed—when the Battery master switch is turned on, DC power is supplied through the closed Avionics Master switch to the avionics bus relay solenoids causing these relays to open and disconnect the avionics buses from DC electrical power. Subsequently selecting the Avionics Master switch to the "ON" position opens the switch which disconnects the DC power to the avionics bus relay solenoids, allowing the relays to default to the closed position thereby supplying DC power to the avionics buses. The checklist action item for the Avionics Master switch power failure is to pull the 5 amp Avionics Master circuit breaker, which electrically opens the avionics master switch circuit thereby restoring DC power to the avionics buses.
See the following circuit diagram:

(Source: own work)
This system works, and works well, by all appearances. However, the design does seem to be somewhat counter-intuitive.
My question is, what is the purpose of designing the avionics master switching system normally closed?
Despite being counter-intuitive, I can understand that the electrical loads required by the avionics demand the use of relays, and that the design allows for the relay solenoids to be powered for a minimal time. I think this is the most reasonable explanation for the design, but I am looking for documented confirmation. Is there some other design consideration that I have overlooked? Does anyone know of a documented reason for this design?
NOTE: While you are welcome to speculate with me on this issue in the comments or chat room, I am not looking speculative answers. I am looking for an answer coming from persons familiar with the King Air electrical system and preferably backed up with documentation, such as the maintenance manual, or other sources that I have overlooked.

Design elements like this generally have to do with failure modes and how you want the system to fail when (not if) it fails. In this case, if there was a momentary or worse, permanent, loss of DC power to the relay or it failed for another reason you would want them to stay closed (and continue in the "ON") position so you would not lose your avionics (provided the power supply was some how still operational to them). Unlike most other electric components (semiconductors) relays are actually electro-mechanical components and they do have moving parts with in them that can fail. In my experience (non avionics mostly) relays have a much lower cycle lifetime than their semiconductor friends.
If we dive even a bit deeper into them, a relay is effectively an electromagnet coil that actuates a small metal rod that generally moves a copper (or similar) strip to close a switch connect. Applying a steady state voltage to a coil to generate a magnetic field (and keep the relay in a desired position) also drives current through the coil. The coils, being very thin gauge wire, act like resistors and can generate a substantial amount of heat. Due to the very thin gauge of the wire this can lead to melting and shorting of the coil. The wire also tends to be resin coated which can heat and melt as well causing a short. In turn you are correct that they don't want to he help on for a long period of time.
This is the electrical equivalent of many mechanically similar ideas in aircraft design. For example some landing gear in planes is held up using positive pressure so should the pumps fail the landing gear may simply deploy or be able to be shaken down by releasing a lock, the idea being its better it can come down in the case of a system failure than be stuck up.

I don't buy the idea that a solenoid would generate damaging heat with prolonged use. The King Air uses a powered solenoid for the battery circuit, both generators, and the inverter in use. Most other aircraft do the same for the master switch.

You might be right that potential failure states are the consideration here, but depending on which component failed that state could vary significantly. Here, the checklist action item for a failed avionics bus is to pull the avionics circuit breaker, which should again open the circuit to the relays and restore power to the buses.

I have seen many relays fail from prolonged use for all kinds of reasons, shorts and melts included. The failure state of a component is for that component only and often assumes all else is working. Chain failure (and redundancy) is an entire aspect of these designs but is beyond the scope of this question.

Pulling the avionics circuit breaker will kill the avionics (as far as I know it will not cycle them unless you close the breaker again). The breaker is in line with the +V on the aircraft and is most likely pre relay. The relay control unit may have its own breaker.

Are you familiar with the King Air avionics bus circuitry? Pulling the Avionics Bus circuit breaker will power the buses. The Avionics Bus circuit breaker protects the avionics switch. There are separate breakers that protect the individual systems off the avionics buses.

I am not that intimately familiar with them, generally pulling a breaker colloquially means removing it from the active circuit and subsequently cutting power, it seems that is the reverse case in your wording.

@JonathanWalters: The CB seems to protect first against a possible short circuit in the self-inducted back current removal diodes, then against a less likely short circuit in the relays, the switch or the wires. The diodes could have been in series with the coils (in the reverse direction, as it is the case usually), but this is not fail safe for normally-on relay, since in case of a broken diode the relay would have been prevented to work.

Dave, regarding the avionics master circuit breaker, this system is "reverse" as you describe. That is, in part, why I am asking the question. I understand very well how the system works; I am looking for a reason for why it was designed this way.

Im pretty sure the why is because that is the desired safety failure state of the system.

Middle of the 2nd paragraph: "The coils, being very thin gauge heat". I'm reasonably certain that should read " The coils, being very thin gauge wire", correct?

My guess:
From a functional and electrical point of view the task could be done by a single, 30A, double-pole, manual switch taking the place of the relay contacts in the circuit.  One chunky switch will probably be just as reliable as a dual-feed bus, 5A switch and two relays.
I reckon that Beechcraft might have chosen this configuration so they could (i) have a lighter, smaller 5A single-pole switch on the panel and (ii) avoid routing an additional four 30A wires to that panel.   This choice becomes even more sensible if a highly-reliable dual-feed bus was already available.
Disclosure: I am an electronics engineer but have no aircraft engineering experience.

That's a good explanation for the use of relays instead of a central switch, but it misses the reason to use normally-close relays rather than usual normally-open ones.


text:
What is the purpose of this aileron trailing edge strip?
See the following photos of the LH aileron of a Beechcraft King Air B200:

Source: own work

Source: own work
As seen in the above photos, the Beechcraft King Air B200 has what I would describe as a bulged trailing edge strip on both ailerons. These strips are roughly centered in the span of the aileron, and in the case of the LH aileron are located outboard of the trim tab.
In figure 57-50-00-06, the Beechcraft B200 IPC lists these as -320 and -325, calling them the LH and RH trailing edge strips:

The King Air 200 has the same trailing edge strips, which are listed as -70 in fig  57-70-10 of its IPC with the same part number as on the B200.
I do believe the King Air 300 series also has these strips, though I am not certain. I don't believe the King Air 90 has these, but again, I could be wrong.
I have asked many King Air pilots and maintenance crew if they knew what the purpose was, with no definite answer. The best answer I have recieved was a tentative suggestion that they might be for something like dampening control buffet.
Does anyone know the purpose for these bulged aileron trailing edge strips? Does anyone know of any documentation for their purpose? Do similar control surface design feature appear on other aircraft?

Related: AOPA Article on the King Air 350 "To boost rudder effectiveness in engine-out situations, a spanwise, airfoil-shaped strip is attached to the rudder’s trailing edge. This “tricks” the rudder into behaving as though its area is enlarged, according to Roger Hubble, Textron’s senior vice president for special missions. A similar strip is on the airplane’s left aileron, as is the case with all other 350s."

@RonBeyer I read that article a while back, and dismissed that quote as having any bearing on the 200 aileron strips since, A) I would hesitate to call the aileron strip "airfoil-shaped", and B) the strip is located on both the left and right aileron on the 200. Quite frankly I forgot about that article. However, there may be something to it; the aileron strip could be "similar" and not truly "airfoil-shaped".

@RonBeyer I suppose AOPA is correct insofar as there is a somewhat similar strip on the left aileron. However, it would seem that any implications either that the similarity extends to function or that the strip is located only on the left aileron would be incorrect

Mr. Tom Clements—the Tom Clements—told me the following in answer to my inquiries on this matter.1 While I cannot yet offer any documentation on this in the form of a Beechcraft publication, this comes pretty close, and is—I think—definitive enough.
According to Mr. Clements, the aileron trailing edge strip serves to re-center the aileron after control surface deflection.
In the early 1970's, Mr. Clements was the first Beechcraft factory instructor in King Air 200. The Project Engineer for development of the King Air 200 was a Mr. Leroy Clay. Prior to achieving FAA type certification for the 200, Mr. Clay was discussing the prototype aircraft with Mr. Clements—Serial Number BB-1—and revealed the following, which I paraphrase from Mr. Clements.
BB-1 borrowed wingtip and aileron design from the late model King Air 90s, including the wingtip which extended beyond the outboard end of the relatively short ailerons. After its first test flight, it was decided that BB-1—with its more powerful engines, greater wingspan, different roll inertia, etc—was lacking in aileron roll authority. The design solution was to extend the ailerons outboard to the wingtips, for a greater overall aileron span.
This change proved to solve the control authority concerns, but revealed a new problem: the ailerons lacked sufficient aerodynamic centering tendency, and now tended to remain partially deflected after a roll control input. The initial solution was the bulged trailing edge strip in question. The idea was that the bulge would cause a small amount of drag, especially when deflected, which would tend to streamline and center the aileron.
I say the initial solution, because, Mr. Clements recalls that Mr. Clay, the Project Engineer, stated that the bulge was a temporary fix added to the aileron, with the intention of a complete aileron redesign to follow prior to certification. I will point out the obvious: it has been over 40 years since the King Air 200 was certified with that bulged trailing edge strip.
Mr. Clements goes on to confirm that this trailing edge strip has remained a feature of King Air models through all the 200 and 300 series.

1This exchange took place and is available on BeechTalk.com, though the content is only accessible to members. BeechTalk.com terms of service do not allow copying and reproducing member content outside the site.

This explanation of the use of the strip on subsequent 200 and 300 models reminds me very much of my sister's habit of always cutting a roast in half and cooking it in two smaller pans. She doesn't know why; she does it because our mother did it. Mother did this because Gran always cooked a roast this way. After some international travel and interpreted interrogation, I was able to learn that Gran cooked a roast in two pans because the oven in her primitive wood stove was not large enough for a proper roasting pan.


text:
What are the requirements to obtain an FAA private pilot's license in Bosnia?
I'm a US citizen currently living in Bosnia. Before I left the states, I was working on getting my PPL.
I currently have about 65 hours and was very close to finishing, but I was unable to do so before I left because of inclement weather and preparing for the move.
Would I be able to finish getting my license in Bosnia?
Are flight requirements the same? Would the flight hours transfer?
If I complete my license here would it be valid in the states?

You would have to get a Bosnian license and then transfer it to the FAA. So first you need to figure out what it takes to get a license in Bosnia.

You might check if it's cheaper/easier to come back to the US for a break, finish your US certificate, then go back to Bosnia and convert or validate it to fly there. And before you commit to a Bosnian license, consider if you'll have to maintain/renew it regularly and what your long-term plans are. FAA licenses never expire, which is a huge plus.

@Pondlife That was my first thought as well.

Your largest obstacle would be finding a FAA CFI and a FAA Flight Examiner in Bosnia.

You would have to find a US-licensed CFI in Bosnia to give you the training, and then a Designated Examiner in Bosnia to give you the oral and practical exams. The first may be doable, the second pretty much impossible unless you want to pay someone to come to Bosnia. Certainly it would be much easier (and probably cheaper) to come back to the US for a few weeks and do nothing but finish your ticket.


text:
Why does IRS alignment take so much time?
Home GPS systems (like your car navigator) is able to locate you in like a few seconds, but 737NG needs to align itself for 6-8 minutes.
Why do navigation systems in aircraft take so long to align themselves?
Full name:
INS = Inertial Navigation System
IRS = Inertial Reference System

Wild guess: Time to spin up gyros vs speed of light.

The AN/WSN-7 RLG used on Navy ships takes days to properly align. Regardless of the align method selected, a previously calibrated system requires between 16 and 20 hours to reach specified full navigational accuracy. A system that has not previously completed calibration requires between 68 and 72 hours to reach specified full navigational accuracy.

Further: The actual time required for the system to settle to within specification accuracy is determined by several factors. These include: geographic position, heading and speed of the ship, time of entry and accuracy of first position reset, the alignment method selected, and whether or not the navigation system has been previously calibrated.

That was posted as a comment because it (sort of) addresses the question, but not really...

If your car navigator is inaccurate, it's not that big of a deal. A quick location fix is usually good enough. If a 737 navigation system is inaccurate, consequences are much more serious.

Not an answer, but I've noticed that my GPS takes a LOT longer to lock in when I shut it down and take it 10,000 km away with the power off. As if it has a memory of where it was and expects to be not that far away when it wakes up.

@SpehroPefhany: that is exactly what happens. Getting a GPS fix "out of the cold" can take up to 30 minutes. This can significantly be sped up if you a) know the time, b) know where you are, c) have a current almanac, d) have current ephemeris data. Which is why mobile phones use so-called aGPS (accelerated / assisted GPS), where the phone gets a) the current time from its internal clock, b) a rough position from the cell carrier (via cell towers) and Google/Apple/Microsoft (via nearby WiFi networks whose location is known), c) and d) downloads the data via the cellular network instead of …

… waiting for the satellite to send it (which may take up to 30 minutes). Also, an automobile GPS unit typically uses additional intertial data to correct for location data, it can generally assume that you will be on a road and not, e.g. a lake, and it will also assume that you follow its directions, so if it tells you to turn left at the next intersection, it will actually show you turning left at the intersection and may only "realize" a couple of seconds later that you actually turned right. You can e.g. sometimes observe your position "jumping" between two parallel roadways.

@JörgWMittag, a nitpick: the "jumping" position is rather caused by the navigation software trying to snap you to a road, even if the real position doesn't really put you right on top of the map's representation of the road. Also, if the navigation software is slow, it might be trying to compensate for its own slowness by assuming what is your next position; this compounds the snapping problem. Anyway, I think it's been some time since I saw these kinds of problems live - I'd say they were more typical of underpowered navigation systems.

GPS faster? Yes and no
GPS is faster depends on which case we want to consider: Cold start, restart or operational use.

Cold start, like for example the first time we use the instrument: Inertial reference systems (IRS) are faster than GPS to deliver the first fix (TTFF): Less than 10 minutes (alignment) against 12.5 minutes for the GPS, the time to receive the constellation almanac from any satellite, a message which is valid for 90 days.

Warm start, like from one day to the next: GPS is faster, on the previous use, it stored the almanac. It now has to get the ephemeris for each satellite in sight, a 12s data transmission repeated every 30s (interleaved with almanac data) and valid for 2 hours. If the restart occurs within 2 hours and ephemerides for satellites in sight where cached, then it takes only a few seconds for the receiver to lock on the satellites data steam and to assess the ionospheric effects on radio signal velocity. IRS initialization takes the same time than for a cold start.

Operational use: IRS is faster, it can deliver a position immediately and continuously when properly aligned, while the GPS may be able to deliver only 10 or 100 updates per second. This parameter is important for fast aircraft, in 0.1s an airliner at M.82 cruises by more than 200m (if you were to use the car navigation system you mentioned in the question, with an update rate of 1s, there would be 2 km between consecutive fixes).

So for day to day operations, IRS is definitely slower to initialize as you observed.
GPS and IRS are not used the same way
We may wonder why these slow IRS are still seen in the plane electronics bay. First, they have two considerable technical advantages:

They are faster to deliver information when aligned. This capability is of prime importance for performance-based operations.

They don't rely on the availability of an external infrastructure to be operational (satellites, monitoring/control stations, SBAS overlays like WAAS, EGNOS, SDCM...) nor on radio transmission. Prior to 2000, the GPS was subject to selective availability under the decision of US generals and it still could suffer from intentional jamming.

But regardless of these advantages, IRS have not the same objectives than GPS. GPS is used to get a position, a Doppler groundspeed and a time reference. Other information, like heading, attitude, linear and angular accelerations, wind vector (with the help of the air data unit), drift angle, all critical data for a large automated aircraft, must be obtained through other sources, the first one being the IRS.
For example the quick determination of attitude parameter is critical for flight envelope protection and yaw damper control. This can only be done with an IRS. When an Airbus aircraft loses its 3 IRS, it loses most of the fly-by-wire protection and the rudder control falls back to mechanical linkage, so the pilot can control yaw with pedals, feel the aerodynamic efforts on rudder and maintain them within acceptable limits, now that yaw rate detection has been lost.
IRS actually provides primary data to various devices like auto-braking (anti-skid), vertical speed indicator or thrust management computer.
This explains why IRS are not going to be removed from electronics bay (it can be INS, IRS, IMU or today combined with air data units like ADIRU). On the other hand as they are subject to continuous drift, their position can be periodically updated by the GPS position (or most likely by a weighted mix of different sources).
Let me detail the principle behind IRS and GPS, and how they are started.
Time to get a fix from an IRS
An IRS is composed of 3 accelerometer-gyrometers pairs mutually perpendicular. Gyrometers quantify angular motions, like yaw or roll, and set a frame of reference for the accelerometers. The accelerometers quantity linear motions to update the aircraft position.

Overall principle of a strapdown IRS, source
During alignment the accelerometers sense the sum of gravity acceleration and centripetal acceleration (due to tangential velocity from earth rotation, proportional to latitude) in order to determine the horizontal plane (from the gravity field), true north (from earth rotation axis) and latitude (from centripetal acceleration). But there is a challenge:

Determining pole axis requires knowing the local centripetal acceleration, which in turn depends on latitude, and

determining which part of the locally sensed acceleration is due to latitude (and not to gravity) requires knowing the direction of earth rotation axis (true north). This is numerically solved using successive approximations of both data.

This process is convergent, and both latitude and north direction accuracy increases as the process is extended in time.

The convergence is slow and reaching an operational accuracy indeed requires the long time we know.

The slowness comes from earth rotation produces only tiny effects on the acceleration sensors, and we haven't found more practical sensors yet. The effects depends on the distance from earth rotation axis, larger at the equator than at the poles, hence alignment is also faster at the equator.
Accuracy of true north bearing increases with alignment time:

Source: An Improved Alignment Method for the Strapdown Inertial Navigation System
Alignment requires the unit to be motionless, any erratic motion decreases IRS signal/noise ratio which slows this process (passengers boarding, wind acting on the tail surfaces, etc).
GNSS principle: Need to know the current position of the satellites
To compute a fix, the receiver must know:

The satellites positions.
The time it takes for the signal from each satellite to reach the receiver.

... both data with an astronomical precision.
To compute the position, the receiver needs the orbital elements computed on the ground, from control and tracking stations measurements, uploaded to the satellites which act only as relays to broadcast this information along with a time reference from a very accurate embedded clock.
The GNSS receiver then computes the satellite current position along its orbit, and determines the time which has been required for the GPS signal to travel in atmosphere, providing the receiver distance to a known point in space. With 4 satellites and 3D triangulation techniques, the receiver position can be determined.
Satellites parameters are broadcast at different rates:

Rough parameters for all satellites, aka almanacs, valid for 90 days, are transmitted by all satellites every 12.5 minutes (so it may take up to 12.5 minutes to a brand new receiver to deliver the first fix).

Complementary precise parameters for a given satellite, aka ephemeris (which include specific corrections), valid for only two hours are transmitted by this satellite only, every 30 seconds, so it may take up to 30s for a receiver with a valid almanac to provide an accurate fix after restart (if the cached ephemerides for the currently visible satellites are too old), even if a rough fix can be delivered nearly instantaneously.

It should be mentioned that these days there is inertial reference system, which provides inertial navigation, but also feeds the attitude indicator, heading indicator, autopilot and, on aircraft that have them, the flight law controllers, all of which also benefit from the alignment.

@JanHudec: Related user manual of an ADAHRS where the synergy between data sources is visible (e.g. air data checked and validated by IMU).

that is also important, yes. All in all, it makes the inertial reference an essential part of the avionics suite rather than just a navigation backup.

@Mins You can change them to lower heading types if you want them to be smaller, but for people using screen readers, they need to actually be headers. Boldface text is not a header. Screen readers identify header markup and use it for navigation.

@T.J.L. The typography I used is part of my choices, as the wording, and I don't want it to be changed (for design reasons). Please respect my intellectual rights on that and remember that I could have not used titles at all (and you wouldn't challenge me). Instead, put pressure on the site designers, so that header sizes are appropriate. FYI, inter paragraph titles are supposed to be either the same size (but e.g. bold), or one point larger than text. That's good typography (and I know what I'm talking about). Your intent is a good one, but your solution must see both problems, not one only.

@mins One of us edited for personal preference, one of us edited for accessibility for a legally recognized disabilities. If you don't like how SE handles typography, you should bring it up with them, because my edit actually helps people read your otherwise good answer.

@T.J.L. I see you're trying to start a discussion, If you want to do that, open a private chat and I'll make an exception to discuss with you.

https://aviation.meta.stackexchange.com/questions/4047/does-personal-preference-take-precedence-over-accessibility

Also to mention is GPS only really tells you where you "were" not where you are.  GPS pulses are only received at the staggering rate of 1 Hz. So once a second you get a "position fix" on where you were one second ago, plus some dead reckoning on apps like Google Maps and such which attempt to guess where you are going between pulses.  INS on the other hand computes almost continuously, with many aviation variant like the Honeywell 764 calculating position at about 240Hz.
Depending on the data interface connected, you won't get the position fixes that quickly (40hz) but that's still much faster than GPS. Plus an INS provides information to an airplane that's almost more important than position, and that's attitude, velocities, and accelerations.  These are absolutely essential for systems like Autopilot, Heads up display, radar, etc.  Plus with a closed loop  Kalman filter, you can ensure that calculated errors are fed back into the system to give you extremely accurate position over a long period of time. (~0.1NM/he drift).
GPS is useless for autoland, autonavigation, and self contained approaches, which are a big deal for military applications.
The Honeywell mentioned above is an EGI (enhances GPS/INS).  It takes the GPS pulses and hacks the INS position every second.  This really is the best of both worlds. When GPS is highly accurate (based on the position of the constellation overhead) then you can't beat a GPS position. But when GPS is bad, or denied, the INS will reject  the hacks and get the job done.
GPS is convenient, but I don't think it'll ever replace a good old fashion INS.

Welcome to aviation.SE. This seems more a comment to another post rather than an answer to the original question. Please consider editing it to meet the standards of this site. Once you will have gained enough reputation, you will be able to comment under any post.

Welcome! That seems to be a (good) comparison between INS and GNSS, but this is not related to why INS alignment is slow. You should have answered this question: Do today's aircraft still have INS (inertial navigation system)?


text:
For "Climb via SID except maintain FL180", what does "except" mean?
Reading the clearance: "Climb via SID except maintain FL180."
What should the pilot do about the FL?

Related: What does "Except" mean in this ATC request?, but in this case it's probably not a typo

Does this answer your question? What does "Except" mean in this ATC request?

This is not a TYPO.

This is not a duplicate of the linked question.

Assuming you are in the usa, this is correct phraseology. It means that you are to comply with all constraints published in the assigned SID—lateral, vertical, and speed constraints—but once you reach the specified altitude or flight level (FL180 in this case) you are to stay there until receiving another climb or descent instruction from ATC. Even after you level off, you are to continue complying with all other constraints from the SID; from that point on you disregard the vertical information only.
From the AIM 5–2–9h:

4. (Using the Example 3 flight plan, ATC determines the top altitude must be changed to FL180). The clearance will read:
“Cleared to Johnston Airport, Scott One departure, Jonez transition, Q-One Forty-five, Climb via SID except maintain flight level one eight zero.”
NOTE-
In Example 4, the aircraft must comply with the Scott One departure lateral path and any published speed and altitude restrictions while climbing to FL180. The aircraft must stop climb at FL180 until issued further clearance by ATC.

See ATC's guidance for this, the 7110.65 4–3–2e4(b) for "full" departure clearances and 4–3–3e2 for "abbreviated" departure clearances:

Instruct the aircraft to “Climb via SID except maintain (altitude)” when a top altitude is not published or when it is necessary to issue an interim altitude.
EXAMPLE-
“Cleared to Johnston Airport, Scott One departure, Jonez transition, Q One Forty-five, Climb Via SID except maintain flight level one eight zero, expect flight level three five zero one zero minutes after departure.”

It means the controller wants you to climb, according to all the instructions in the SID, except maintain the altitude that they gave you, as opposed to the maximum listed on the SID.

If it is not a typo then pilot should maintain FL180. A SID may allows flight to a point higher than FL180 but ATC wants the plane no further than 180.

Typos are rarely preceded by the word "except"...

Edit…..
Apparently it’s not a typo. As a non-FAA pilot, I think it is a ambiguous and requires clarification.
What should the pilot do? First they need to understand the clearance and then read it back correctly.
This is how I would respond: “Climb via the SID, maintain FL180. Confirm that all altitude restrictions are cancelled and that we should climb directly to FL180 and maintain FL180”
The controller will respond affirmative,  or give further clarification to this POORLY worded clearance.

The clearance was not "cleared via SID, maintain FL180." The clearance was "cleared via SID and also climb via SID, except maintain FL180." This is neither a typo nor a lazy controller; it is acceptable phraseology in the USA.

It may be acceptable phraseology in the USA, but I think it is ambiguous and requires clarification.

That is why the FAA publishes the AIM, to provide clarification. If you are unfamiliar with the AIM then by all means please ask rather than making an assumption, but the correct answer to OP (assuming they are not currently sitting in an aircraft copying their IFR clearance!) is not "ask" but rather "check the AIM."

Does the FL180 limitation supercede my EFC+10 altitude?  If I have a comms failure, do I proceed to FL180, then proceed to FPA or Expect +10 alt 10 minutes from departure? Or would I have to complete the trip at FL180?

@John that would better be a separate question, but: The "except maintain" is part of your initial altitude instruction. "Climb via SID except maintain FL180" is all one chunk. The "expect" altitude will be issued separately, whether by voice or as information printed on the SID, and it is to be complied with separately. So in the case of lost comms you would still evaluate your "expect" altitude as part of the decision.


text:
Why do delta wings generate less lift at a particular angle of attack compared to straight wings?
I simulated a delta wing and a straight wing in xflr5 with the same profile, wing area and airspeed (very low airspeed. around 10 m/s. it's for a model aircraft). The straight wing was operating at nearly the lift coefficient of the 2d airfoil at a particular angle of attack, but the delta wing was operating at a lift coefficient far below the 2d airfoil coefficient. Why is this so? The wing has a medium sweep of 25 degrees. Will the lift improve with a higher sweep angle?

Many elements in Why are delta wings not commonly used in commercial airliners? answers, and What are the advantages and disadvantages of a delta wing compared to a swept wing?

Hello! the main thing I found in the above answers was on how much drag they produce in subsonic flight. My question has nothing to do with drag. One point mentioned was that they require higher takeoff and landing speeds. why is that so?. I need an explanation for all of these points mentioned in previous answers based on aerodynamics.

I believe this particular answer provides some reasons: high sweep angle leading to lift decrease for the same AOA. For the new question (higher speeds) you may want to edit the post to include it.

If this reduction in lift is due to sweep, then shouldn't a normal swept wing with the same sweep angle have such reduction in lift? But this is not so. The lift reduction is much less compared to the delta plan

No, a higher sweep angle will reduce lift even more.
All aerodynamic forces scale with the dynamic pressure, the product of air density and the square of airspeed, divided by half. If a wing doesn't produce lift, move to denser air or speed up.
However, drag will also scale with dynamic pressure, and there is more which influences lift. One is the angle of attack and the second is the suitability of the chosen wing geometry to produce lift at a given speed and angle of attack. Straight wings score best in this respect, at least at subsonic speed.
The sweep angle of the quarter-chord line gives you a first approximation of lift loss. Lift is proportional to the cosine of that angle. Just drive sweep to its extreme of 90°, and the inability of such a wing to create lift should become clear.
Lift is produced by the downward deflection of the air streaming around a wing. When the wing meets this flow straight on, it will have the biggest effect on the air and create the most lift. Any sweep, regardless of forward or backward, will reduce the wing's effect.
The next effect is the influence of the wingtips. If your wing extends sideways a lot more than it extends in flow direction, the influence of the tips is relatively lower. The ratio between wing span and chord is called the aspect ratio, and it figures prominently in the formula for the lift curve slope.

So the inability of the delta planform to generate lift at a specific airspeed and AoA and area compared to its similarly specced straight wing counterpart is because of a mixed effect of lower aspect ratio and wing sweep?

@Sherlock_Dumbledore: Yes, exactly.

I know this was answered a while back but let me give you my two cents.
As explained by Peter a combination of the wing swept and low aspect ratio affect the lift production of the wing. But another effect also takes place; vortex lift. aeroalias explains it very nicely on this post: What is vortex lift?
In short vortex lift is lift generated by the vortex sheets that accelerate air downwards. And as you said the lift generated by a delta wing is lower at the same angle of attack, but a delta wing is able to go to much higher angles of attack than traditional aircraft until the vortex 'bursts' and the delta wing stalls.

This provides great performance on delta wings when traveling supersonic, but when landing they must land at a higher angle of attack. This is the reason for which the Condorde had a drooping nose. Since it was landing at such a high angle of attack, the engineers had to add it to meet with regulations regarding the ability of the pilot to see the runway.


text:
What is the average angle planes wings they need to fly?
The planes I am talking about are everyday passenger plane's, not fighters. Do you know what the angle needs to be for it to create enough lift so the plane can fly?

Do you mean Angle of Attack?

I do. Thank you

That helped a lot. thank you.

The angle of attack airplanes fly at while cruising is called the optimum angle of attack (around 4 degrees) which meets the max.(L/D).

Thank you! I appreciate it.

@TrentAlexanderArnold you're most welcome.


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What is this aircraft parked in Oceanside Municipal Airport?
I was on my way to the beach at Oceanside on October 2020. On 4:32, I saw multiple Cessna or Beechcraft aircraft, as well as a Velocity V-Twin, on the runway of the Oceanside Municipal Airport.
When I was looking at the further spots, I noticed an odd-looking, single-engined twin tail aircraft parked on the runway. As seen in this photo, the red arrow indicates the strange aircraft.

That’s an Ercoupe.  No longer manufactured but still many examples flying today.

It appears this one was identifiable based on the pic alone, but thanks for providing all the additional time & location info. Usually, that's necessary, and most have to be prompted to provide it.

"the red arrow indicates the strange aircraft"  Hmm, I was really wondering what that red arrow meant.

Most likely an ERCO Ercoupe. BTW, this tail configuration is called H-tail.
Note the similar-looking Max Holste MH.52, but with the noticeably smaller dihedral angle.

The Forney F-1 variant on wiki is exactly the same, even the color!

@bentoy13, That's because Forney bought the manufacturing rights & tooling from ERCO's successor and manufactured Ercoupes for a while under the Forney name.

That's almost certainly an ERCO Ercoupe! It's a beautiful plane, and it actually has a fairly unique characteristic that makes it quite safe- it's characteristically IMPOSSIBLE to spin! IIRC the ailerons and rudder are both controlled by the yoke- the aircraft is always coordinated! It's a cool plane even when you don't consider that feature, but with that system in place, I think it's one of the neatest GA planes out there!


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Why are some item numbers missing in ICAO flight plans?
I was looking at an ICAO flight plan and observed that there are only specific ITEMS mentioned in it (see example below)? What is the reason behind excluding the item numbers 1, 2, 3, 4, 5, 6, 11, 12, 14 and 17?

(source: FAA)

Technically, item number 3 is on the flight plan: it is the message type (which is fixed at FPL for flight plans).

@Bianfable Could also be RPL or FPT (and possibly others)

From the FAA page:

Item numbers on the form are not consecutive, as they correspond to Field Type numbers in ATS messages

Here you can find a description of ATS messages.


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Why is the VOR reference and variable signal modulated at 30 Hz?
VOR signals radiated from the ground beacon are modulated at 30 Hertz.
What is the reason behind this?

This is related to the use of the FFT algorithm for decoding. You can look at this document of nist https://www.nist.gov/system/files/documents/calibrations/tn1069.pdf section 16.1, but well, not so easy to explain.

@mins, you should post this as the answer. It's a good one.

In VOR early days, the 40s, the stable phase locked loop frequency generator was not available. A common frequency reference was the very stable public AC power grid frequency. The angular velocity of a synchronous motor is locked on the power source frequency, such motor can be used to actuate a variable resistor or capacitor to generate a stable wave which frequency is locked on the power supply frequency.
A VOR relies on two 30Hz signals: Reference signal and variable signal (more on the VOR principle here). The US grid is 60Hz, a synchronous motor with two pairs of poles turns at 1800 rpm, that is 30 rounds per seconds.
The reference modulated the frequency of a 9,960Hz subcarrier (332x30Hz) with a modulation index of 16, meaning the subcarrier swinged between 9,480Hz and 10,440Hz. Here is the description of the mechanical assembly used:

A tone wheel is driven by an 1800 rpm (30 rps) motor; and on this
wheel are 332 teeth which are arranged in a slightly staggered manner
so as to impart a cyclical variation between 9480 Hz and 10,440 Hz
with the rotation of the motor. The frequency modulated signal from
this tone wheel is then used to amplitude modulate the VOR carrier
signal.

(Source: A comparative analysis of area navigation systems in general aviation, by Stephen Malcolm Dodge, NASA -CR-132504, 1973.)
The motor also drove the rotating dipole broadcasting the variable signal:

Early conventional VOR with rotating beacon, source
Here's what the rotating/modulating assembly looked like for a Collins VOR station:

Motor, tonewheel and dipole of a Collins conventional VOR, 1954. Source: Collins
A single motor used to generate both the reference and the variable signal ensured the two signals remained coherent, and therefore allowed a simple phase angles subtraction to get the bearing.
If the VOR had been designed in Europe, the same motor would have turned at 25 rps, and the signals frequency would have been 25Hz instead of 30Hz. There comes @Sanchises question below, about the consequences of the different grids on the VOR compatibility around the world... While the motor angular velocity is the first to be impacted by a different power supply frequency, harmonic interference must also be considered, for example:

In the ILS there are two reference frequencies at 90 and 150Hz, also chosen as multiples of 30 and remote enough from harmonics of line frequency (60, 120, 180Hz, ...). However 50Hz harmonics include 150Hz and interferences may occur on a 50Hz grid.

Modern solutions to solve both the angular velocity problem and the harmonic interferences problem are straightforward, but they were not 60 years ago. It seems designers at this time chose to recreate a 60Hz source locally and dismissed 50Hz AC for the related portion of the equipment. I got this schematics from a 1968 issue of the French aeronautical navigation engineering group bulletin:

Mechanical generation of 90Hz and 150Hz signals in an early ILS. Source: STNA bulletin, page 88
A 1,800 rpm synchronous motor is used to generate mechanically the two frequencies. But France has opted for a 50Hz grid. A static inverter is used as a local 60Hz power supply. As mentioned in the source document, the inverter frequency is stabilized with a 1% accuracy.

Did early European VORs then just use a gearbox?

@Sanchises. I included the answer at the end of the post.


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How is a kit plane certificated if it does not meet the 51 percent rule?
I'm reading up on building an airplane, and it says the builder must accomplish 51 percent of the work to get an Experimental Amateur Built airworthiness certificate. But what happens if a certain type of plane doesn't meet the 51 percent rule, how is it certificated then?

"Experimental" is a term for homebuilts only used in the US.

FWIW, AFAIK all kit planes sold in the USA meet the 51% rule.  The only way you wouldn't is if you somehow got the kit vendor to do a significant assembly or set of assemblies for you.  The one possible concern would be if you purchased a partially completed kit plane, and I read about that being done enough that there seems to be a way to deal with it.

I think you're asking about FAA regulations? If so, please edit your question to clarify, including adding the faa-regulations tag. This question might be relevant.

Seems as though the problem is with defining what goes into "the work".  If you buy a complete engine, or avionics package, instead of building one from parts, you've saved yourself a lot of work.  How would that be different from e.g. buying completed wing assemblies?

@Bianfable of course.  My point was the term experimental when used on that context makes it clear the OP is in the US.

@JohnK Got it. I added the FAA tag.

If you don't comply with the 51% rule as explained in AC 20-27G - Certification and Operation of Amateur-Built Aircraft the FAA will not issue an Experimental-Amateur Built airworthiness certificate.
What happens after that up to you and what the FAA determines is the specific reasons for non-compliance. If the reasons can be fixed and it brought into compliance, they can eventually grant the certificate. If not, you would have to pursue certification under another category, most likely Experimental-Exhibition. The downside is that will come with significant restrictions. That will have a significant negative impact the value of the aircraft. There's also no guarantee that it can be certificated at all.
As @ZeissIkon said, all US kits are 51% rule compliant, so the risk is when you get 'Builder Assistance' in the terms of the FAA. Follow the guidelines and you'll be fine.

It is still certified "experimental", you just can't apply for or receive the repairman certificate, so you need to treat it just like a certificated aircraft when it comes to annual and repairs.

Without knowing what country's regulations apply, how do we know for certain?  Or are you confident this applies in every jurisdiction on Earth?

That experimental cert won't be the same as homebuilt 51% "amatuer built" experimental cert.  It'll be under one of the other experimental categories and will come with a lot more restrictions.  A lot of people questioned the CAA's (pre-FAA) use of that term "experimental" for amateur builts in the early 50s due to the confusion it created, but  it was done as an expedient to speed up the regulatory process at the time, and it's stuck.

@ZeissIkon Given the OP's other questions are all US/FAA specific, I extrapolated that and the "51% rule".

@JohnK That's a good point, it's "experimental" but not "amatuer built". There are a number of restrictions, such as flying near built-up areas, at night/VFR only, etc.

Here in Canada they used to call homebuilts "Ultralights" until the 70s. Then ultralight ultralights came along and confusion reigned.  It was changed to "amateur built" in the 80s.  In some ways the rules are looser here than the US.  Ex; there is no Repairment Cert requirement in Can for homebuilts.  Whoever's name is on the C of R has authority to sign off on all maintenance, repairs, and annuals.  Only ELTs and Xponders 2 yr encoder recerts need an approved shop sign off.


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How does a synchro resolver converter work?
There are Synchro converters or transducers used on aircraft to measure angle of attack using 3 sinusoidal inputs. I could not understand where  these 3 inputs come from and what angle information is generated by the synchro converter on the basis of the input signals?

The angle of attack sensor turns the shaft on a synchro (or a resolver: they're essentially the same thing) which is a transformer whose outputs are uniquely related to that shaft angle. That's where your 3 inputs come from. There are lots of possibilities for what happens next. The shaft angle is not the same as the angle of attack. The synchro is not measuring. It's just conveying measurement information from the sensor to the flight crew and other aircraft systems.


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What is the purpose of aircraft localization?
I want to understand what is the purpose of doing localization while an aircraft is flying? What sensors are used by the instruments that do this localization of an aircraft? Is there more than one way to do the localization? Can we do localization without using a GPS? How is magnetic heading used to do localization?

Can you clarify what you mean by "localization" - I understood it as "what is the purpose of knowing where you are?" - which seems kinda obvious.

I think it's meaning is related to navigation and current location of an aircraft in air.

When my father flies his Piper Cub, without any electronics at all, including no radio, his localization is usually "looks out the window" :)

Are you asking about the localizer portion of an ILS approach?  I am guessing that you heard this term and are using it in your question, but not in the correct context.  If you just want to know what a localizer is there is plenty of information out there.  Or you could just ask that.  As is, the question needs clarification and I am voting to close.

I want to understand what is the purpose of doing localization while an aircraft is flying?

Knowing where you are means you won't get lost and arrive at your destination instead of literally anywhere else.

What sensors are used by the instrument that does this localization of an aircraft? Are there more than one ways to do the localization? Can we do localization without using a GPS? How does magnetic heading is used to do localization?

These are all highly related questions. Yes, there's more than one way to do it and most ways don't involve GPS. The "instrument" is usually a pilot combining the different sensors, or a flight computer in more advanced airplanes.

Looking out the window and checking with a map. Suffices for most VFR flying. A compass can help with orientation.
GPS. Quick and easy, but requires pretty expensive instrumentation. A much cheaper handheld tablet may be used to help with VFR navigation.
Radio guides. VOR and NDB can be used to see what bearing you are relative to beacons on the map. Using more than one, you can triangulate where you are. VOR/DME can be used to get your position with just one beacon. Requires a map with known beacons. ILS can help you get to the runway.
Inertial navigation system (INS). Extremely sensitive equipment register all accelerations, and by integrating twice determine position. Errors add up over time, so other sources can be used to periodically correct for this.

I want to understand what is the purpose of doing localization while an aircraft is flying?

Well, in general it would be nice to know where you currently are, wouldn't you agree? Even aircraft flying under VFR (Visual Flight Rules) should be able to determine their location, either by looking out of the window and identifying landmarks they know or by tuning in VORs. Knowing where you are is obviously important for finding your way to your destination airport, but it also helps to find a suitable diversion airport in case of an emergency.
Most IFR (Instrument Flight Rules) flights today are capable of RNAV (Area Navigation), which means they navigate based on knowing their own position rather than following ground based radio signals (like VORs or NDBs). This obviously implies that they need to have accurate localization.

What sensors are used by the instruments that do this localization of an aircraft? Is there more than one way to do the localization? Can we do localization without using a GPS?

There are multiple ways to determine the aircraft's position. The oldest standalone system is an INS (Inertial Navigation System). This system is aligned on the ground and you need to enter the current position. It will then keep track of the aircraft position by integrating all rotations and accelerations by measuring them with gyroscopes and accelerometers.
Such INSs are part of a modern airliner's ADIRU (Air Data Inertial Reference Unit) today. Because of inaccuracies in the measurements, the inertial position will start to deviate from the true location of the aircraft over time, which is called drift. This drift can be corrected by a radio based update (e.g. DME/DME), where the location is determined with radio based signals. Today, it is more common to use a GNSS (like e.g. GPS) to do these drift corrections. GPS is however not required for the ADIRU to work.
Smaller aircraft typically do not have an INS. Many modern GA aircraft have a GPS unit installed, which will provide location. For these units, GPS is required, and if it should fail, the location would be lost. The pilot can then still use conventional navigation (e.g. VOR to VOR) to navigate.

How is magnetic heading used to do localization?

The magnetic heading is just the direction in which the aircraft is pointing (using magnetic rather than true North). This alone will not give you any localization.


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Do pilots of commercial aircraft have ready access to a map or data showing nearest airports in case of a need to make an emergency landing?
As a passenger I often wonder that if a pilot had to make an emergency landing, do they have access to live information to direct them to the nearest airport or is this something that they have to rely on from air-traffic controllers?  I'm thinking that seconds could mean the difference between life and death and having information as such on-board could be a life saver, rather than rely on others to figure out where to go during a crisis.

Since you asked about commercial aircraft, I will answer for a modern airliner: The Boeing 777. The details will differ between aircraft models, but the general functionality should be available in every modern airliner.
With a modern airliner equipped with an EFIS (Electronic Flight Instrument System), there is no need to carry paper maps with possible diversion airports any more. Selecting a suitable alternate and navigating there can be done with the FMC (Flight Management Computer).
On the 777, the FMC has a page with alternate airports that are selected automatically from the navigation database, but can also be overridden by an uplink (where the airline dispatch sends suitable alternate airports to the aircraft) or manually (e.g. required for ETOPS flights with selected ETOPS alternates). This page looks something like this:

(Boeing 777 FCOMv2 11.43.21 - Flight Management, Navigation - FMC Descent and Approach)
The list of four airports is shown in 1 and the estimated time of arrival (ETA) as well as the fuel at the diversion airport is shown in 4 and 5 respectively. 2 allows a request of alternate airports from the airlines dispatch via ACARS. In order to divert to one of these airports, you select the respective line from 1 and the press DIVERT NOW (9). This will set up the flight plan with a direct line towards the selected airport.

Alternate Airport Diversions
ALTN page 1/2 data aids the flight crew in finding a suitable alternate airport.
The page displays up to four airports in ETA sequence. Each airport on the list has an XXXX ALTN page with more data. [...]
Three alternate airport uplinks can be received. ALTN LIST page 1/2 can get an uplink for the entire page or just the ALTN INHIB line. ALTN LIST page 2/2 can receive an uplink of alternate airport names.

(Boeing 777 FCOMv2 11.43.20 - Flight Management, Navigation - FMC Descent and Approach)
These four diversion airports can also be displayed on the Navigation Display (ND):

(Boeing 777 FCOMv2 10.10.35 - Flight Instruments, Displays - Controls and Indicators)
This is particularly helpful if adverse weather is in the area (e.g. thunderstorms) because the weather radar would depict areas with large amounts of precipitation on the ND as well. Should an alternate diversion airport be out of display range, it will be shown like this:

Displays the FMC or pilot selected primary alternate airport. Displays up to four alternate airports when the EFIS control panel ARPT map switch is selected on.
Offscale airports (those beyond 640NM with 640 scale selected), directional arrow relative to the airplane position or PLAN center point and distance (distance remove in PLAN).

(Boeing 777 FCOMv2 10.40.20 - Flight Instruments, Displays - Navigation Displays)
All of this allows a fast decision for an alternate diversion airport. Nonetheless, communication with ATC (Air Traffic Control) is of course essential in an emergency and ATC will assist in finding a suitable diversion airport if necessary. I don't quite agree with

I'm thinking that seconds could mean the difference between life and death [...].

If you only have seconds to make such a decision, it means you are at very low altitude, meaning either just after takeoff from your departure airport or already very close to your destination airport. In this case, the usual choice in case of an emergency is to return to the departure airport or continue to the destination. There are exceptions of course, like e.g. a takeoff alternate:

A take-off alternate aerodrome should be specified in the operational flight plan if either the meteorological conditions at the departure aerodrome are below the applicable landing minima or in case it should not be possible to return to the departure aerodrome for some other reason.

(SKYbrary - Alternate Aerodrome)
In this case the takeoff alternate would however be chosen before even departing, so no quick decision is necessary.

Much appreciated, thanks.  This level of sophistication increases my feelings of security while flying.  I generally feel quite safe flying over land, knowing that there is usually an airport nearby in case of an emergency.  But flying over water always worries me.  Cheers.

@BeginnerBiker for concerns about flying over water, do some investigation into [tag:ETOPS] (Extended-range Twin-engine OPerationS). There are a boat... er... plane-load of rules and regulations about how far you can be from a diversion airport based on the individual aircraft you're in and what that particular aircraft is certified for.

Generally, yes. The FMS will have a database, updated every few weeks, that contains all possible navigational data that one might need within the selected region(s). This is needed so that pilots are able to enter any possible route they might need, including ATC rerouting them or diverting to other airports due to weather or emergencies.
I don’t know airliner FMSes in particular, but even GA planes have a single button that will show you all the nearest airports and key data like runway lengths, and you can select and start navigating directly to any of them in one or two clicks specifically to address the exact scenario you mention.
There are even apps for phones and tablets with the same capabilities; they aren’t legal for IFR navigation, but VFR pilots use them for that every day, and IFR pilots do use them for other reasons or at least have them in their bag as an emergency backup.
Since these databases are large and rather expensive, you don’t load data that clearly isn’t needed. For instance, a US domestic carrier would still load the Canadian regions since they might overfly it between US cities, but they wouldn’t load regions on other continents since their planes couldn’t get there anyway. And GA pilots might only load the region they live in.

Maps and charts are always available in all aircraft flying for scheduled commercial carriers. They used to be in the big leather flight cases that pilots used to drag through the airports. General Aviation aircraft are not required to have maps and charts for VFR flight. But, it is highly recommended. Regardless, each pilot in command, before beginning a flight, is required to become familiar with all available information concerning that flight. This may include emergency landing options and alternate airports. Technology has made it a lot easier to have charts and maps available as well as having all information concerning each flight.
In a “glass panel” aircraft, maps and charts are loaded into the aircraft’s onboard navigation system. These maps and charts can be geo-referenced to provide a moving map showing the aircraft’s location based on GPS. In addition, pilots can now use apps loaded on their tablets with the capability of storing maps and charts of the entire world. Although, just the US would take anywhere from 30-50 gigs including the app software itself.
In an emergency where landing at an airport is an option the nearest airport can be found fairly quickly. Most glass panels and tablets have a nearest function that will direct the pilots to airports fitting preselected parameters. These parameters are usually based on airports able to accommodate the aircraft based on runway dimensions, available instrument approaches, etc. For reference, the nearest functions of the Garmin G1000 are shown below. One of which is activated by the push of one button on the Primary Flight Display.
Luckily, the higher the aircraft is flying, the further it can glide to a suitable airport for landing. This may take a surprisingly long time from over 30,000 feet. If the aircraft is in an uncontrolled and rapid descent, it may be pointless to attempt a landing at an airport no matter how near it is.
One such flight that landed successfully was Air Transat Flight 236 that glided without power for 75 miles in almost 20 minutes.


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What is the best time to run runway friction tests?
When is the best time to run runway friction tests?
And what is the optimum temperature to run them?

"optimal" w.r.t. which parameters/constraints?

The friction will change over time due to temperature and moisture and other factors.
So, the best time to measure is shortly before a plane is about to land. In order to allow planning of alternatives, allow a longer time.

I found a couple of FAA sources on this, I don't know if other countries have similar guidance or not. The FAA information covers two reasons to measure runway friction: for maintenance, and for operations in winter (ice etc.).
For maintenance, the information is in AC 150/5320-12C - Measurement, Construction, and Maintenance of Skid Resistant Airport Pavement Surfaces. The PDFs seem to be corrupt for me (missing and blank pages), but the draft version looks better. It recommends basing inspections on the minimum number of turbojet aircraft landings per runway end each day; see table 3-1 on page 8. The frequency ranges from a minimum of once per year for less than 15 landings up to once per week for more than 210 landings.
Because that's a draft and the published version doesn't work correctly for me it's possible that the published values are different.
For winter operations, the guidance is in AC 150/5200-30D - Airport Field Condition Assessments and Winter Operations Safety. There's no strictly defined frequency that I could see although it does mention four situations in which the airport should do a friction survey (section 5-2-1):

There's a specific amount of ice or snow on the runway, and the operator "believes the information will be helpful"
The runway centerline is contaminated for a distance of more than 500ft
"Following all snow clearing, anti-icing, deicing, or sanding operations"
Immediately after any incident or accident on the runway

Both ACs have a lot more information in them.